In a wireless communications system comprising of multiple devices sharing a common RF channel to transmit data, a medium access protocol governs the which peripheral has access to the RF channel at a specific moment of time and for a specified duration. Many medium access protocols are distributed where each peripheral operates as a peer performing identical algorithms to access the channel. In other protocols, one device may act as the master receiving requests for the channel access and granting service. In all these, systems their is opportunity for the RF transmissions from two peripheral devices to occur simultaneously, colliding and destructively interfering such that no information may be communicated. The medium access protocol must reduce the probability of collision to maximize the data transfer rate of the system as whole.
The amount of data each peripheral device has to transfer varies with time. Peripherals are generally bi-modal and exist in active or standby mode. Peripheral devices are active when they have data to send and inactive when no data is awaiting transmission. When active, the peripheral device accesses the channel transmits the data and returns to a standby mode. The probability of collision is a function of the number of peripherals in the system and the arrival rate of data to send.
A simple method for reducing the collisions is by not having each peripheral transmit with absolute certainty in the active mode. Rather, each peripheral will transmit with a fixed probability. This probability is known as a persistence level. It has been shown, that persistence level can significantly reduce the probability of collision when the number of devices and their respective arrival rates are known. However, a fixed persistence level appropriate for a given number of devices may become sub-optimal if the number of devices or arrival rates change.
Most systems also employ a busy-tone or carrier sensing to improve the data transfer rate by insuring that once one peripheral device has seized the channel, no other peripheral device will begin transmitting. This method reduces the probability of collision by shortening the period that peripheral devices may contend for access to the system. Once peripheral device has seized the channel all other devices will defer transmission until that peripheral completes. During that transmission, many peripherals will transition from the standby to active mode and be ready to transmit data. The probability of collision immediately following a transmission is greatly increased. Collisions following the transmission compound the problem allowing a larger number of peripherals to transition to the active mode starting a catastrophic spiral where the newly active peripherals further contribute to the collisions such that no data is successfully transmitted.
In a multipurpose communication system, the channel may be shared by other types of transmissions which cause the channel to appear busy and the phenomenon described may occur whenever the channel transitions from busy to ideal.
A random back-off algorithm is nearly always employed to avoid the catastrophic cycle following the busy period. In a random back-off algorithm each peripheral device detects if the channel is busy or if there has been a collision and the waits a random period, called a back-off period, before attempting transmission. Subsequent collisions may require that the peripheral back-off for longer period which each successive collision. This type of algorithm is effective, however, it is complex. It requires that each contending peripheral maintain has a memory of the previous collisions requiring a complex state machine.
maymaymay Thus there is a need for a method, an access point device and a peripheral device for providing low complexity dynamic persistence for random access by a peripheral device in a wireless communication system.